In order to diagnose a power storage device mounted in a vehicle (including a so-called battery or electric double-layer capacitor), it is known to use measurement of an internal resistance. For example, JP 2014-230343A discloses a technology in which a main battery and a sub battery are provided together, and the internal resistance of the sub-battery is measured using the current value and the voltage value at the time at which a smoothing capacitor is charged by the sub-battery.
It is also known that, when diagnosing the power storage device, higher accuracy can be gained by using not only the internal resistance, but also the voltage of the power storage device at the time of opening (open-circuit voltage). However, if the vehicle in which the power storage device is mounted is traveling, the power storage device is charged by an alternator provided in the vehicle, and thus a charging path from the alternator to the power storage device needs to be cut off when the open-circuit voltage is measured.
On the other hand, when supplying power to an electric load mounted in the vehicle, there is a case where a discharge current from not only the alternator but also the power storage device is required while the vehicle is traveling. Also, because this discharging path has a portion in common with the above-mentioned charging path, there is a possibility that the supply of power to the electric load is not sufficiently performed if the charging path is simply cut off.
In view of this, an object of the present invention is to provide a technology that avoids a power shortage while a vehicle is traveling caused by measuring an open-circuit voltage of a power storage device mounted in the vehicle.